This section provides background information related to the present disclosure which is not necessarily prior art.
Switched-mode power supplies typically operate at high switching frequencies ranging from a few KHz to MHz range. There are typically periodic voltage changes on the terminals of a switched-mode power supply transformer and corresponding voltage swings on every winding layer of the transformer. Common mode noise coupling typically occurs as a result of voltage differences between one winding layer and its neighboring layers. The voltage differences drive AC current through the inter-winding capacitance of winding layers, causing coupled noise current. The net common mode noise current coupled between windings, and most importantly between primary and secondary windings, is typically non-zero for most power supply designs.
Common mode noise is typically electrical noise measured between a power supply and an earth ground reference point. Common mode noise may occur for a number of reasons, including the number of turns of neighboring windings being different, a phase relationship between neighboring windings being out of phase, a non-zero inter-winding capacitance between layers, mechanical alignment of layers, insulation thickness variation between layers, phase relationships of different topologies, etc.
Several techniques may be used to counter common mode noise, including electromagnetic interference (EMI) filters, differential and common mode inductors, block and bypass EMI components, etc. Circuit to earth capacitors (e.g., Y-capacitors) can provide local bypass of interference current and reduce externally measureable interference components. Transformer winding shields may be used to block and bypass inter-winding coupling components completely, partially, etc. EMI noise cancellation may also be used.
Common mode noise may be coupled through a transformer to a user device. The common mode noise may interfere with touch pad capacitive sensing, result in sluggish response in screen movement, etc. Smaller size and costs of some charger applications for smart phones, tablet computers, etc. may not allow for the use of conventional common mode filters for interference attenuation.